Preparation Of VOCl₂ Photocatalysts And Its Photocatalytic Performance For Selective Oxidation Of Cyclohexane

The efficient selective oxidation of cyclohexane to cyclohexanone and cyclohexanol（KA-oil）remains one of the most challenging research projects in catalytic chemistry.Photocatalytic technology has become an ideal reaction route because it can utilize renewable solar energy to achieve high-efficiency and high-selective oxidation of low carbon alkane at room temperature and with air or oxygen as an oxidant.In this thesis,V₂O₅/Ti O₂ and VOCl₂ photocatalysts were prepared and applied to the photocatalytic cyclohexane selective oxidation reaction under mild conditions.The structure,light absorption properties,electrochemical properties of photocatalytic materials and the effects of different factors on the performance of photocatalytic oxidation of cyclohexane were investigated.A series of supported metal oxide catalysts including V₂O₅/Ti O₂,Ag/Ti O₂,Fe₂O₃/Ti O₂,Cr₂O₃/Ti O₂ and In₂O₃/Ti O₂ were successfully prepared by equal volume impregnation method,and characterized by powder X-ray diffraction（XRD）,X-ray photoelectron spectroscopy（XPS）,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）,N₂-adsorption desorption and UV-Vis diffuse reflectance spectroscopy（UV-Vis）to analyze the composition,morphology and light absorption properties of the catalysts.Also,the effects of metal oxide types,different solvents,vanadium content and acid additives on photocatalytic reaction were investigated.Studies have shown that the V₂O₅/Ti O₂ photocatalyst has higher performance for selective oxidation of cyclohexane to KA oil among these metal oxides;the reaction solvent has an important influence on the activity and selectivity of the photocatalytic reaction,when the reaction was performed in an acetonitrile/water mixed solvent,the photocatalytic activity and selectivity were the highest;when HCl was used as an additive,the photocatalytic activity was greatly improved.Under this condition,the cyclohexane conversion rate was 4.9%,and the selectivity to the KA-oil product was close to 100%.In addition,according to the results of active species capture experiments,the possible reaction mechanism of V₂O₅/Ti O₂ photocatalytic oxidation of cyclohexane was proposed.VOCl₂ photocatalyst was developed for the first time and used in selective oxidation of cyclohexane to cyclohexanol and cyclohexanone by molecular oxygen under mild reaction conditions.The effects of catalyst concentrations,reaction times,different solvents,different acid additives and oxygen pressures on photocatalytic performance were investigated.The results show that VOCl₂ has high photocatalytic activity in pure Me CN solution,and a cyclohexane conversion of 17.8%with ca.99%selectivity for KA-oil can be obtained after light irradiation of 4 h.It is worth noting that the use of hydrochloric acid as an additive can effectively enhance the photo-catalytic oxidation of cyclohexane,providing a cyclohexane conversion of 23.3%,which may be due to the important role of HCl in improving the light absorption,decreasing the oxidation-reduction potential of V⁵⁺/V⁴⁺and promoting the redox process of photocatalyst.Increasing the oxygen pressure and reaction time favors the conversion of cyclohexane and the selectivity of cyclohexanone,but the KA-oil selectivity was reduced due to the deep oxidation of cyclohexanol and cyclohexanone.A possible reaction mechanism for the photocatalytic oxidation of cyclohexane was proposed according to the UV–vis spectra,EPR and photocatalytic activity.